Single-bit audio streams can be advantageous over multi-bit audio streams as being simpler in concept and execution. Single-bit audio streams are frequently used for conveying high definition audio. For example, the pulse density modulation (PDM) is used for high-quality audio in the Super Audio CD (SACD) format. However, single-bit audio streams, such as PDM, can have a data-dependent edge rate that produces undesirable playback artifacts. Edge rate refers to the rate of transitions of the audio stream from a ‘1’ to a ‘0.’ An audio stream with a data-dependent edge rate has an undetermined timing of transitions from a ‘1’ to a ‘0.’ The data-dependent edge rate artifacts can include inter-symbol interference in current-steering digital analog converters (DACs), resulting in poor total harmonic distortion and noise (THD+N). Current-steering DACs are common components for reproducing audio in mobile phones and other audio devices. Thus, it is desirable to convert a single-bit audio stream with a data-dependent edge rate to a single-bit audio stream with a constant edge rate. An audio stream with a constant edge rate has a transition from a ‘1’ to a ‘0’ within determined times.
One conventional solution for obtaining a constant edge rate audio stream from a PDM audio stream uses a return-to-zero (RTZ) format and is shown in FIG. 1. A system 100 upsamples a single-bit audio signal at block 102. The upsampled signal includes multiple copies of each bit of the single-bit audio signal at a higher data rate than the signal-bit audio signal, such that the upsampled audio signal has redundant information. RTZ block 104 receives the upsampled audio signal and inserts a gap between symbols (groups of bits) to obtain at least one up (‘0’ to ‘1’) and one down (‘1’ to ‘0’) transition in every symbol. However, with a current-steering DAC, each bit of the modified single-bit audio signal controls the addition of current to the output of the DAC. The insertion of the gap in RTZ block 104 changes the analog output generated from the modified single-bit audio signal and attenuates the signal. This introduces undesirable noise in the reproduction of audio in the single-bit audio stream. Furthermore, the introduction of the gap undesirably changes the modulation index of the audio stream.
Still further, there is limited control over the volume of the audio signal being reproduced from the single-bit audio stream or the modified single-bit audio stream. Conventionally a modulator is used for volume control on digital signals. However, modulators affect the quality of the reproduced audio.
Shortcomings mentioned here are only representative and are included simply to highlight that a need exists for improved electrical components, particularly for audio devices employed in consumer-level devices, such as mobile phones. Embodiments described herein address certain shortcomings but not necessarily each and every one described here or known in the art. Furthermore, embodiments described herein may present other benefits than, and be used in other applications than, those of the shortcomings described above.